Fumaric acid, also known as trans-butenedioic acid or (2E)-But-2-enedioic acid, is a compound with the chemical formula HO2CCH=CHCO2H. It has a white crystalline structure and is key to organic acid biosynthesis in mammals and plants. Fumaric acid is one of the strongest food acids, used in pie fillings, animal feed, medicines, and cleaning agents.